1. Field of the Invention
The present invention relates to a near field scanning optical microscope for imaging a surface of a sample to be examined.
2. Description of the Related Art
As a conventional near field scanning optical microscope, a microscope is known in which light from a light source is radiated onto a sample to be examined through a pin hole or aperture formed in the distal end of an optical probe, and light transmitted through the surface of the sample or fluorescence generated by the sample itself is focused by an objective lens of the microscope (refer to O plus E by S. Okazaki, September 1989, pp. 110-116 and so on). It is known that, in a near field scanning optical microscope of this type, the limit of its resolution substantially corresponds to the diameter of the pin hole or aperture.
When the diameter of the pin hole is decreased in order to improve the resolution, however, the energy radiated on its near or far field is abruptly decreased.
In comparison to a case in which the pin hole diameter is 5 nm, when the diameter is decreased to 1/2, the amount of light reaching a photodetector is calculated to be decreased to about 1/60. That is, when the pin hole diameter is decreased in order to improve the resolution, the amount of light reaching the photodetector is abruptly decreased, separation of signal and noise becomes difficult, and the lower limit of signal detection is restricted. In practice, the resolution is restricted in accordance with the lower limit of detection.